The present invention relates to a cryogenic refrigerator, and in particular to a cryogenic refrigerator of Gifford-McMahon cycle type.
Conventional cryogenic refrigerators of the type, disclosed in U.S. Pat. No. 2,966,035, include a cylinder within which a displacer is slidably fitted. At opposite end portions of the displacer within the cylinder, there are defined an upper chamber and a lower chamber, respectively. An upper end portion of the displacer is connected to a lower end portion of a rod and an upper end portion of the rod is extended in a casing. The casing is adjacent to the upper chamber but is separated fluidically therefrom. A motor is operatively connected via a crank to the rod so as to move the displacer cyclically.
However, the motor has to move the rod against force applied thereto. Such force F is so large, as is apparent from the following formula, that the motor has to be one with high power. EQU F=cross-sectional area of the rod.times.(pressure of fluid in the upper chamber-pressure of fluid in the casing)